Eye muscle dysfunction (strabismus) that affects horizontal movement of one or both eyes creates an inward or outward deviation, while disorders of the muscles that move the eyes up and down produce a vertical, and occasionally a rotational, ocular muscle imbalance. The ocular misalignment and accompanying double vision (diplopia) may result from direct or indirect damage to one or more of the cranial nerves (III, IV, VI) that innervate a particular extra-ocular muscle or muscle group (paralytic strabismus, as may occur following head trauma); as a consequence of direct involvement to the affected muscle itself (restrictive strabismus, for example in a patient with thyroid eye disease); or as a combination of the two etiologies (an orbital fracture where ocular contents, including muscles, are trapped in the fracture site, accompanied by an eye muscle paralysis of the trapped muscle). It is essential for eye care professionals to perform various tests that identify the involved muscle(s) and the type of deviation present, and quantify the amount of deviation for documentation purposes and to assist in planning a course of treatment.
Several methods exist to test for eye muscle dysfunction. One, the Hess test, utilizes a tangent screen consisting of a gray, wall-mounted board. A grid of coordinate curved horizontal and vertical lines appears as a virtual sphere. The patient is seated 0.5 meters from the screen with their head immobilized by a head/chin rest. Because the test is dependent upon color dissociation, the patient wears goggles of red and green complementary filters, red over the right eye and green over the left.
Another method, the Lancaster Red-Green test, is a variation of the Hess test and has similar components: it consists of a calibrated tangent screen, originally printed or sewn onto a piece of dark gray material. The nine diagnostic positions of gaze are marked on the screen, each 22.5 degrees or 45 prism diopters away from the center. The patient is seated 1 meter from the screen, and the head is similarly immobilized. The test utilizes the same goggles as the Hess test, but both the patient's and examiner's flashlights include a cover with a slit such that the light projected onto the screen is in the shape of a bar.
In older versions of the Hess screen the examiner holds a flashlight that projects a dot of red light onto the screen at the intersection of a coordinate. The patient, wearing the red-green goggles, holds a flashlight that projects a green linear target and subjectively superimposes it on the examiner's red dot. The examiner then moves the projected red dot until all nine (9) diagnostic positions of gaze have been evaluated. Newer electronic models have replaced the examiner's handheld flashlight and incorporated point red lights that randomly illuminate at each coordinate, allowing the examiner to observe the test procedure and document the patient's responses. The test is completed after the colored lenses are worn over each eye so that both right and left visual fields are plotted.
With the Lancaster Red-Green Test, both the red and green targets are linear, and again goggles of red and green complementary filters are worn by the patient, with the red filter worn over the right eye. An examiner must still be present to project one of the flashlights. The patient is asked to superimpose a green line projected from his flashlight onto a red line projected on the test screen from the examiner's flashlight. Again, the examiner moves the projected line of red light until all nine diagnostic positions of gaze have been evaluated. Horizontal, vertical, and torsional deviations can be identified and quantified after the patient's responses are correlated to the examiner's target placement. After the test is completed with the right eye fixating, the flashlights are exchanged and the test repeated so that the left eye assumes fixation.
Both the Hess and Lancaster Red-Green tests are fovea-to-fovea tests: the subjective visual direction of each fovea perceives the image seen through each colored filter, but is visually unaware of the image from the opposing eye. The test responses correspond to the direct projection of each fovea, and therefore correlate with the type of deviation present. A patient with a “crossed eye” (esotropia) will indicate that the images are crossed, while a patient who presents with an outward deviation of the eyes (exotropia), will perceive the images as uncrossed.
Since horizontal deviations are caused by problems affecting lateral movement of one or both eyes, these deviations are best appreciated if the green line from the patient's flashlight is projected so that a vertical line is created, while vertical deviations require the line to be projected in a horizontal fashion. While both the Hess and Lancaster Red-Green tests can be used to identify horizontal and vertical derivations, the presence of torsion, a rotational deviation where objects in the vertical meridian are seen as tilted, is difficult to assess with the Hess test because the two test objects—a dot of red light illuminated on the test screen and a line of green light projected from the flashlight held by the patient—are dissimilar in shape.
To score the Hess test, the examiner records the patient's responses on a paper chart, and then connects the dots, which form inner and outer grids. The inner grid measures deviations of approximately 15 degrees, or 30 prism diopters, the practical fields of eye movements from the primary position when the head is immobilized. The outer grid represents deviations of approximately 30 degrees or 60 prism diopters, when head movement is allowed to accompany the movements of the eyes.
The Lancaster Red-Green test also uses a grid for recording patient responses. A single grid sheet has two separate imprinted images of the test's tangent screen, one above the other, implying fixation with each eye. Responses are plotted on the top grid, as first the right eye fixates and then the bottom grid, for fixation with the left eye.
Once the dots are connected, the resulting grids from each test are interpreted by the examiner to reveal the etiology of the ocular misalignment. The grids from the Hess test implicate the affected eye (indicated by the smaller field), the associated under- and over-action of muscles, and may delineate a paralyzed muscle from a restricted one. With the Lancaster Red-Green test, the interpretation of the resulting grids and measurement of the deviation depends on the distance between the red and green lines, as well as the presence of horizontal, vertical or rotational separation of the lines. The field of greatest separation identifies the affected muscle(s) or the greater deviation created when the eye with a restriction fixates; the displaced direction of the patient's line—horizontal, vertical and/or rotational—indicates which horizontal and/or vertical muscles are involved.
While the Hess and Lancaster Red-Green tests aid in the detection of paretic extraocular muscle palsies and of strabismus, their use has been limited by the need for a physician or technician to record the results on an examination chart. Not only does this add to the cost of administering the test, but it introduces a source of error. Furthermore, while the Hess and Lancaster Red-Green tests use point sources or symmetric fixation points, they are unable to capture the torsional component of an ocular motility disease.